The invention relates to a process for producing composite articles in resin transfer molding systems using, as a one component catalyzed resin, a stable liquid epoxy resin composition containing a latent curing agent.
Resin transfer molding (RTM) and vacuum assisted resin transfer molding (VARTM) are processes for fabricating fiber-reinforced composite articles. The processes involve two basic procedures; 1) fabricating a fiber preform in the shape of a finished article and impregnating the preform with a thermosetting resin, commonly called a matrix resin. The resulting fiber-reinforced composite articles display high strength and low weight. Such articles are used in the aerospace industry and for other applications requiring low weight, high strength materials.
The first step in the RTMNARTM processes is to fabricate a fiber preform in the shape of the desired article. The preform generally comprises a plurality of fabric layers or plies that impart the desired reinforcing properties to a resulting composite article. Once the fiber preform has been fabricated, the preform is placed in a closed cavity mold. The mold is closed and the matrix resin is injected into the mold to initially wet and eventually impregnate the preform. In RTM systems, the matrix resin is injected under pressure into the mold. The matrix resin is cured to produce the final composite article.
In a VARTM system, the preform is covered by flexible sheet or liner. The flexible sheet or liner is clamped onto the mold to seal the preform in an envelope. A catalyzed matrix resin is then introduced into the envelope to wet the preform. A vacuum is applied to the interior of the envelope via a vacuum line to collapse the flexible sheet against the preform. The vacuum draws the resin through the preform and helps to avoid the formation of air bubbles or voids in the finished article. The matrix resin cures while being subjected to the vacuum. The application of the vacuum draws off any fumes produced during the curing process.
The matrix resin in RTM and VARTM systems must possess a very low injection viscosity to allow complete wetting and impregnation of the preform. Typically, RTM and VARTM systems employ "two component" epoxy resin compositions wherein the hardener and resin components must be combined immediately prior to use. Common "one component" epoxy resin compositions must be stored at controlled low temperatures to prevent crosslinking reactions and to extend storage life.
U.S. Pat. No. 5,369,192 discloses epoxy resin-based compositions that may be used as matrix resins in RTM systems. The matrix resins shown therein comprise at least one aromatic polyepoxide, a fluorene-containing epoxide, and a 9,9-bis(aminophenyl)fluorene curing agent. The matrix resin is a heavy paste that must be heated in order to be injected into the RTM system. The resin impregnated preform must be heated to at least 350.degree. F. to cure the epoxy resin-based composition. There is a need for a stable composition having a low viscosity at room temperature and that is curable at temperatures between 200 and 300.degree. F.
British Patent Application 2,263,479 discloses liquid compositions that can be used for resin transfer molding including at least one nitrogen-containing glycidyl compound and a Lewis acid. Examples 5-7 describe a mixture of diglycidyl ether of bisphenol A and a Lewis acid. Each mixture was initially cured at 80.degree. C. and postcured at 200.degree. C. to produce polymers having a glass to rubber transition temperature up to approximately 100.degree. C.
An article entitled "A New High Temperature Epoxy System for RTM Application" by A. Wang et al. similarly describes the use of a tetraglycidyl methylene dianiline epoxy resin in combination with an aromatic amine in RTM systems. The article further describes the use of a tetraglycidyl methylene dianiline epoxy resin and diamino diphenyl sulfone curing agent.